What the square measures
The square is a separation of ninety degrees, a quarter turn of the circle. Neptune's orbital period of about a hundred and sixty five years therefore places the first one in the fortieth year or thereabouts, and puts the opposition well beyond a human lifetime at roughly eighty two.
This is a longitude measurement and nothing more. Neptune is far too faint to see without a telescope, and none of that affects the geometry, which is calculated from an ephemeris in the ordinary way.
Neptune was only identified in 1846, which means the first complete orbit since its discovery has not yet finished. The positions used here are computed rather than observed across a full cycle, which is standard practice and accurate to well under a degree.
Why the age drifts either side of forty
Neptune's path is close to circular by the standards of the outer solar system, but it is still an ellipse, and its distance from the Sun varies enough to change its apparent speed through the zodiac. Crossing a quarter of the circle from one starting degree is not the same length of task as crossing it from another.
In practice the square arrives between about thirty eight and forty two. That is a narrower window than the Uranus opposition produces and a great deal narrower than Pluto's, but it is a window rather than a date.
Because everyone born in a given stretch of years shares roughly the same natal Neptune sign, this transit arrives for a whole cohort at about the same time. What differs between charts is the house and the degree, not the era.
Retrograde passes at Neptune's speed
Neptune is retrograde for roughly five months a year and covers only a couple of degrees of the zodiac in a year of forward motion. That combination makes multiple passes very likely: three crossings spread over the best part of a year is the ordinary outcome, and the planet can sit within a degree of exactness for months on either side.
Because of that slowness, the difference between the exact date and the surrounding period matters less here than for a faster body. The widget above still lists each crossing separately.
Some astrologers therefore date the transit by the window rather than by the crossings, taking the period from when Neptune first comes within a degree to when it finally leaves. That produces a span of a year or two rather than a set of dates.
The traditional reading and its limits
Neptune is associated with dissolution, imagination and the gap between an idea of something and the thing itself. The square is conventionally described as the point where an assumption made in youth stops holding its shape, and modern writing often pairs it with the Uranus opposition as the second half of a midlife account. Disillusion is part of that description, as is a tendency to mistake a fog for a revelation. It is convention, not evidence, and this page forecasts nothing.
House placement is where the reading becomes yours, and the house needs a birth time. Neptune moves so little in a day that the dates are safe without one.
Since Neptune sits in the same sign for a whole generation, anything read from the sign alone applies to everyone born in those years. The house is the part that is yours, which is another reason the birth time is worth chasing down.
Questions people ask about this
What age is Neptune square Neptune?
Usually between thirty eight and forty two, most commonly around forty. Where your natal Neptune sits in its orbit decides the exact year.
Why is there no Neptune opposition for most people?
Because half of a hundred and sixty five year orbit is over eighty years. The opposition would arrive in the eighties, and the return not at all within a lifetime.
How long does this transit last?
Neptune covers very little ground in a year, so the exact crossings are typically spread across the better part of a year and the planet stays close to the angle for months either side.
Is this the midlife crisis?
It is one of the transits that popular writing folds into that idea, along with the Uranus opposition near the same age. The label is a modern shorthand rather than a claim the older tradition makes.
Do I need a birth time?
Not for the dates, which depend on a degree that hardly changes across a day. You need one for the house, and the house is what makes any interpretation specific.
What does the tradition say it brings?
A revision of something taken on faith, described in terms of disillusion and clearer sight. That is an interpretive convention, and the checkable material is the timing and the house.